The present invention relates to transmitters of the type used in the process control industry. More specifically, the present invention relates to a transmitter having an analog-to-digital converter with adaptive resolution, update rate or resolution and update rate.
Transmitters in the process control industry typically communicate with a controller over the same two wires over which they receive power. A transmitter receives commands from a controller and sends output signals representative of a sensed process variable back to the controller. A commonly used method is a current loop where the sensed parameter is represented by a current varying in magnitude between 4 and 20 mA.
A transmitter includes a sensor for sensing a process variable which is controlled by a process. The sensor outputs an analog signal which is representative of one of several variables, depending on the nature of the process to be controlled: pressure, temperature, flow pH, turbidity, gas concentration, etc. Some of the variables to be controlled have a large dynamic range, such as in a flowmeter, where amplitude of the analog signal changes by a factor of 10,000.
An analog-to-digital converter in the transmitter converts the analog sensor signal to a digital representation of the sensed process variable for subsequent analysis in the transmitter or for transmission to a remote location. A microprocessor typically compensates the sensed and digitized sensor signal and an output circuit in the transmitter sends an output representative of the compensated process variable to a remote location.
Frequently, transmitters use a sigma-delta analog-to-digital converter in which the analog sensor output is connected to a high speed sigma-delta modulator. The sigma-delta modulator provides a high speed digital bit stream output representative of the polarity and magnitude of the sensor output. This bit stream is applied to a digital filter such as a decimating filter which generally performs a filtering or integration operation and provides a multi-bit output (i.e., a data byte or a data word) at a slower rate.
Generally, there is a tradeoff between resolution and update rate (or bandwidth) in analog-to-digital converters. The resolution of the analog-to-digital converter is inversely related to the conversion rate (or bandwidth) of the converter.